BATTERY SYSTEM AND METHOD FOR CONTROLLING LITHIUM ION BATTERY

A battery system includes a battery that is a lithium ion battery including an electrode assembly containing a positive electrode active material. An ECU calculates a deterioration index value ED corresponding a degree of progress of high rate deterioration, and when the deterioration index value ED exceeds a threshold value, controls a power converter or a PCU to cause a voltage of the battery to fall within a voltage range including a specific voltage. The specific voltage is a peak voltage on a dQ/dV voltage characteristic curve, the peak voltage being derived from structural change of the positive electrode active material. The dQ/dV voltage characteristic curve is a curve indicating a relationship between dQ/dV that is a ratio of a change dQ of a stored electricity amount to a change dV of the voltage of the battery, and the voltage of the battery.